1. Field of the Invention
The present invention relates to a solenoid-operated valve which is responsive to electric current supplied to a solenoid or coil so as to linearly control fluid flow from a high-pressure passage into a low-pressure passage, and more particularly relates to a linear solenoid-operated valve for use in a speed-responsive power steering system for motor vehicles which is capable of controlling a steering force in response to variables such as vehicle speed.
2. Discussion of the Background
In general, in a speed-responsive power steering system in which a steering force is controlled in response to variables such as vehicle speed, a linear solenoid-operated valve is used which linearly controls fluid flow from a high-pressure passage to a low-pressure passage.
FIG. 1 illustrates two separate flow patterns which occur in a conventional linear solenoid-operated valve similar to U.S. Pat. No. 4,471,811 and which comprises a valve housing 10 having a bore 11 in which there is axially slidably fitted a cylindrical spool 20 having at a lower end thereof slots 21, 21 extending diametrically thereacross. Valve housing 10 has a first passage P1 opening into bore 11 and a second passage P2 extending coaxially from bore 11. Slots 21 serve as bypass passageways 22 that respectively extend between first and second passages P1, P2 and which are throttled as spool 20 moves axially within bore 11. Namely, bypass passgeways 22 are defined with side walls and end wall 21 and a side wall of first passage P1.
In such linear solenoid-operated valve, when the velocity of fluid flow through slots 21 is rapid, pressure acting on bottom surfaces 21a of slots 21 becomes comparatively small in accordance with Bernoulli's equation. Therefore, when pressurized fluid flows at time t.sub.1 from first passage P1 to second passage P2, a first reaction force F1 acts on spool 20 in a downward direction in FIG. 1, so as to throttle bypass passageway 22. When the pressurized fluid flows reversely at time t.sub.2 from second passage P2 to first passage P1, a second reaction force F2 also acts on spool 20 in the same direction. In addition, second reaction force F2 is greater than first reaction force F1 because of the difference in the directions of fluid flow. As a result, an imbalance or unevenness exists between the first and second output pressures or steering forces which are respectively generated when the steering wheel is turned clockwise and counterclockwise, as shown by the double-dot-and-dashed line in FIG. 5.
In order to diminish the aforementioned imbalance between the first and second steering forces, a linear solenoid-operated valve is utilized wherein an end plate D having an orifice D1 formed therein is attached to the lower end of spool 20 as shown by the double-dot-and-dashed line in FIG. 1. In such linear solenoid-operated valve, a first thrust force is added to first reaction force F1 when the fluid flows from first passage P1 to second passage P2 because the pressure which acts on an upper end surface of spool 20 becomes greater than the pressure which acts on a lower end surface of end plate D according to a pressure drop across orifice D1. To the contrary, a second thrust force due to the inverse pressure drop across orifice D1 serves to counteract second reaction force F2 when the fluid flows from second passage P2 to first passage P1, so as to cancel the difference between first and second reaction forces F1, F2.
However, some difficulties have been encountered in fully cancelling the imbalance between first and second steering forces, which is respectively generated upon clockwise and counterclockwise rotation of the steering wheel, when merely providing the orifice. One problem in such linear solenoid-operated valve is that when the fluid flows from first passage P1 to second passage P2, the degree of stagnation of fluid at the end walls 21a of slots 21 changes significantly in accordance with the variation in the degree of opening of bypass passageway 22 and as a result, first reaction force F1 changes a greater amount according to the position of spool 20 than second reaction force F2. Another problem is that when the fluid flows from second passage P2 to first passage P1, the lower portion of spool 21 is elastically outwardly deformed by the pressure introduced into the interior of spool 20, whereby the resistance to the sliding motion of spool 20 varies in accordance with the variation of fluid pressure.